Cellular radio systems, particularly those specified by the Third Generation Partnership Project (3GPP) have evolved to provide data services of increasingly higher speeds. Increased usage of high speed services (for example, due to the proliferation of smartphones) has increased the strain on various parts of the network. An example of such high speed services are the High Speed Packet Access (HSPA) functionalities provided for UMTS architectures (sometimes referred to as 3G) individually in both downlink and uplink. 3GPP Technical Specification (TS) 25.308 and TS 25.319 define the high-level design specifications for High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA) respectively.
The uplink control channels are required to support these high speed services in both downlink and uplink. The signalling load on these channels is high and, particularly in high user load situations, this may lead to spectral inefficiency and an increased cost per bit of useful user-plane data. Conversely, reducing the uplink control channel overhead would increase the data capacity in each cell and make the spectrum and cell site use more efficient.
Referring to FIG. 1, there are shown a number of data frame structures for existing control channels in a UMTS cellular radio network. At the top of FIG. 1, there is shown the data frame structure for the Enhanced-Dedicated Physical Control Channel (E-DPCCH). This is used to send information to the Node B that aids it in decoding the corresponding uplink Enhanced-Dedicated Physical Data Channel (E-DPDCH). The information includes: the Retransmission Sequence Number (RSN); an Enhanced-Transport Format Combination Indicator (E-TFCI); and a happy bit. The RSN is used to identify data that is transmitted multiple times but encoded differently each time. The Node B uses the RSN to establish how to decode the data each time. The E-TFCI is the transport format of the data sent on the EDPDCH. The happy bit provides an indication of whether the mobile terminal has the possibility to send more data. In other words, it indicates whether there is more data in the mobile terminal's buffer and the power available to send it.
The Dedicated Physical Control Channel (DPCCH) includes: pilot bits; a Transport Format Combination Indicator (TFCI); a Feedback Indicator (FBI); and a Transmit Power Control (TPC) command. The pilot bits are used for channel estimation and the TPC commands are used to instruct the Node B whether the mobile terminal requires the corresponding DPCCH or Fractional Dedicated Physical Channel (F-DPCH) power in the downlink to be increased or decreased.
It may be seen that the E-DPCCH and DPCCH each comprise 10 information bits and are spread to 2560 chips. This defines one slot, three slots forming a sub-frame of 2 ms duration. Five sub-frames define a single 10 ms radio frame.
The High Speed Dedicated Physical Control Channel (HS-DPCCH) provides feedback information to allow the Node B to control the High Speed Downlink Physical Shared Channel (HS-DPSCH) that is transmitted in the downlink. This channel comprises: a Hybrid-Automatic Repeat Request (HARQ) Acknowledgment (ACK); and Channel Quality Information (CQI). The HARQ ACK information informs the Node B if the data sent on the HS-DSCH has been received, by sending ACK (acknowledged), NACK (not acknowledged) or DTX (no ACK or NACK sent). The CQI indicates the downlink channel conditions observed by the mobile terminal to the Node B. The HS-DPCCH is one sub-frame in duration, as indicated in FIG. 1. Further details regarding the channel formats may be found in 3GPP TS 25.211, TS 25.212, and TS 25.321.
All of the information data items in these uplink control channels are currently understood to be essential to the operation of services being provided, including the high speed data service on the uplink and/or downlink. Nevertheless, reduction in overheads can lead to an increase in capacity. An existing approach to reducing overheads is Continuous Packet Connectivity (CPC). This acts to reduce the periodicity of transmission for control channel information in the uplink, specifically for mobile terminals that are not receiving, not sending data or neither receiving nor sending data. This thereby reduces the level of control overheads over the radio interface.
Nevertheless, the data frame formats that are used remain the same. Moreover, and this solution is not applicable for those mobile terminals that are sending data, receiving data or both. Further reduction in these overheads, whilst still ensuring that the system can operate robustly, effectively and without requiring significant structural changes to the system design is a significant challenge in the improvement of the network.